Method and apparatus to produce pulp using pre-hydrolysis and Kraft cooking

Abstract

A pulp cooking system including: a cellulosic material feed system, a pre-hydrolysis reactor vessel and a Kraft cooking reactor vessel. The feed material system includes a steaming chip bin and a high pressure transfer device. The pre-hydrolysis reactor vessel maintains the feed material in a mildly acidic condition and allows hydrolysate to be extracted through screens below a hydrolysis zone in the vessel. A wash zone is below the screens and allows wash liquid to flow through the feed material in a cross-current direction. The wash liquid and hydrolysate removed from the feed material is extracted through the screens. The feed material is maintained in a mildly acidic condition through the pre-hydrolysis reactor vessel until the material enters the Kraft cooking vessel where the feed material is treated with alkaline cooking liquors.

The claimed invention is: 
     
       1. A method to produce pulp using a feed system, a prehydrolysis reactor vessel and Kraft cooking vessel, the method comprising:
 steaming cellulosic fibrous organic feed material in a material feed system; 
 adding to the feed material liquor extracted from the prehydrolysis reactor vessel before the feed material enters the prehydrolysis reactor vessel; 
 pressurizing the feed material from the chip feed system in a high pressure transfer device before the feed material enters the prehydrolysis vessel and transferring the pressurized feed material to an upper inlet to the prehydrolysis reactor vessel; 
 subjecting the feed material to a hydrolysis reaction in a hydrolysis zone in the prehydrolysis reactor vessel, the feed material and a liquor flow concurrently in the hydrolysis zone, wherein hydrolysate is generated by the hydrolysis reaction; 
 extracting a hydrolysate-containing liquor through a screen in the prehydrolysis reactor vessel into a conduit, wherein the screen is adjacent a lower portion of the hydrolysis zone; 
 separating the extracted hydrolysate-containing liquor into at least a first portion and a second portion by forming at least a first flow path and a second flow path in the conduit; 
 discharging the first portion of the extracted hydrolysate-containing liquor in the first flow path from the prehydrolysis reactor vessel such that the discharged first portion does not return to the prehydrolysis reactor vessel; 
 returning the second portion of the extracted hydrolysate-containing liquor in the second flow path to one or more elevations of the hydrolysis zone upstream of the screen; 
 adding a wash liquid to a wash zone in the prehydrolysis reactor vessel; 
 as the feed material flows downward past the screen and through the wash zone, washing the feed material with the wash liquid flowing upward through the wash zone to the screen; 
 discharging the washed feed material from the prehydrolysis reactor vessel and transporting the washed feed material to an upper inlet to the Kraft cooking vessel; 
 changing the pH of the washed feed material to alkaline after the feed material is in the Kraft cooking vessel, and 
 Kraft cooking the washed feed material in the Kraft cooking vessel to produce the pulp; 
 wherein the steps of the method are performed continuously and simultaneously as the feed material flows through the feed system, prehydrolysis reactor vessel and Kraft cooking vessel. 
 
     
     
       2. The method of  claim 1  further comprising discharging the pulp from the Kraft cooking vessel to a blow tank, and discharging the pulp from the blow tank to a brown stock washer. 
     
     
       3. The method of  claim 1  wherein the step of pressurizing the feed material includes pumping the feed material through at least one centrifugal pump, which comprises the high pressure transfer device. 
     
     
       4. The method of  claim 1  wherein the step of adding the wash liquid includes injecting the wash liquid from a center pipe coaxial to the prehydrolysis reactor vessel and having a discharge port at a lower region of the wash zone. 
     
     
       5. The method of  claim 1  wherein the hydrolysis reaction is an autohydrolysis reaction and a temperature in the feed material is between 150 degrees and 175 degrees Celsius. 
     
     
       6. The method of  claim 1  wherein the pH of the pressurized feed material in the hydrolysis zone is maintained at a pH in a range of 3 to 4. 
     
     
       7. The method of  claim 6  wherein the pH is maintained at a pH of 4. 
     
     
       8. The method of  claim 1  wherein the wash liquid enters the wash zone at a temperature at least 10 degrees Celsius below a hydrolysis temperature in the hydrolysis zone. 
     
     
       9. The method of  claim 1  wherein the wash liquid includes a bisulfite compound. 
     
     
       10. The method of  claim 1  wherein wash filtrate or liquor extracted from the feed material at a first elevation of the prehydrolysis reactor vessel is reintroduced to the feed material in the prehydrolysis reactor vessel at a second elevation which is above the first elevation. 
     
     
       11. The method of  claim 1  wherein alkaline white liquor is added to the feed material as the feed material is in an upper region of the Kraft cooking vessel. 
     
     
       12. The method of  claim 1  wherein alkaline white liquor is added to the feed material as the feed material is in a transfer line between the prehydrolysis reactor vessel and the Kraft cooking vessel. 
     
     
       13. The method of  claim 1  wherein the extracted hydrolysate-containing liquor is returned directly to an upper portion of the hydrolysis zone upstream of the screen, the liquor is returned to a portion of the hydrolysis zone after the feed material has entered the prehydrolysis reactor vessel. 
     
     
       14. The method of  claim 1 , wherein the pressuring of the feed material occurs downstream of the addition of the prehydrolysis liquor to the feed material. 
     
     
       15. A method for pulping a comminuted cellulosic fibrous organic feed material in a feed system, a prehydrolysis reactor vessel and Kraft cooking vessel, the method comprising:
 steaming and adding filtrate to the feed material as the feed material flows continuously through the material feed system, wherein the filtrate is extracted from the prehydrolysis reactor; 
 pressurizing the feed material flowing from the chip feed system in a high pressure transfer device; 
 moving the pressurized feed material into the prehydrolysis reactor vessel; 
 subjecting the feed material to a hydrolysis reaction in a hydrolysis zone in the prehydrolysis reactor vessel, the feed material and a liquor flow concurrently in the hydrolysis zone; 
 extracting a hydrolysate-containing liquor formed in the hydrolysis zone and the wash filtrate through a screen proximate to a lower region of the hydrolysis zone in the prehydrolysis reactor vessel, the hydrolysate-containing liquor is extracted into a conduit; 
 separating the extracted hydrolysate-containing liquor into at least a first portion and a second portion by forming at least a first flow path and a second flow path in the conduit; 
 discharging the first portion of the extracted hydrolysate-containing liquor in the first flow path such that the extracted first portion does not return to the prehydrolysis reactor vessel; 
 returning the second portion of the extracted hydrolysate-containing liquor in the second flow path to one or more elevations of the hydrolysis zone upstream of the screen; 
 adding a wash liquid to a wash zone in the prehydrolysis reactor vessel, wherein the wash zone is below the hydrolysis zone in the prehydrolysis reactor; 
 moving the feed material continually downward through the hydrolysis zone and the wash zone of the prehydrolysis reactor vessel; 
 moving the wash liquid continually upward through the wash zone to the screen; 
 discharging the feed material from a lower outlet of the prehydrolysis reactor vessel, wherein the lower outlet proximate to a bottom of the wash zone; 
 moving the discharged feed material into the Kraft cooking vessel; 
 adjusting the pH of the discharged feed material to alkaline while the feed material is in the Kraft cooking vessel, and 
 cooking the feed material in the Kraft cooking vessel to produce pulp; 
 wherein the steps of the method are performed contentiously continuously and simultaneously as the feed material flows through the feed system, high pressure feeder, prehydrolysis reactor vessel and Kraft cooking vessel. 
 
     
     
       16. The method of  claim 15  further comprising discharging the pulp from the Kraft cooking vessel to a blow tank, and discharging the pulp from the blow tank to a brown stock washer. 
     
     
       17. The method of  claim 15  wherein the hydrolysis reaction is an autohydrolysis reaction and a temperature in the feed material is between 150 degrees and 175 degrees Celsius. 
     
     
       18. The method of  claim 15  wherein the pH of the pressurized feed material in the hydrolysis zone is maintained at a pH of between 3 and 4. 
     
     
       19. The method of  claim 18  wherein the pH is maintained at a pH of 3.5 to 4. 
     
     
       20. The method of  claim 15  wherein the wash liquid enters the wash zone at a temperature at least 10 degrees Celsius below a hydrolysis temperature in the hydrolysis zone. 
     
     
       21. The method of  claim 15  wherein the wash liquid includes a bisulfite compound when added to the wash zone. 
     
     
       22. The method of  claim 15  wherein wash filtrate or liquor extracted from the feed material at a first elevation in the prehydrolysis reactor vessel is reintroduced to the feed material in the prehydrolysis reactor vessel at a second elevation which is above the first elevation. 
     
     
       23. The method of  claim 15  wherein alkaline white liquor is added to the feed material as the feed material is in an upper region of the Kraft cooking vessel. 
     
     
       24. The method of  claim 15  the extracted hydrolysate-containing liquor is returned directly to an upper portion of the hydrolysis zone upstream of the screen, the liquor is returned to a portion of the hydrolysis zone after the feed material has entered the prehydrolysis reactor vessel. 
     
     
       25. The method of  claim 15 , wherein the pressuring of the feed material occurs downstream of the addition of the wash filtrate to the feed material.